Answer is
1. The central idea or topic is often found in the first sentence and then repeated in the conclusion
2. Read the article, and pay attention to the text to determine the subject of the article. Ask yourself, “What is the subject of the article, and what does the author reveal about the subject?” That's the central idea!
3. Summarize the article in your own words, what is the main idea?
4. Look for repetitions of an idea